/Engine/Plugins/Runtime/MeshModelingToolset/Source/ModelingComponents/Public/Mechanics/RectangleMarqueeMechanic.h
OnDragRectangleFinished is called once the user lets go of the mouse button after dragging out a rectangle. bCancelled flag is true when the drag finishes due to a disabling of the mechanic or due to a TerminateDragSequence call, rather than a normal drag completion.
typedef TMulticastDelegate_TwoParams< void, const [FCameraRectangle](API\Plugins\ModelingComponents\Mechanics\FCameraRectangle) &, bool bCancelled > OnDragRectangleFinishedEvent